Help Ressurecting an Odyssey - geometry and effective mass?

[AndrewL]

Hi All,

A couple of years ago, I bought a prototype Odyssey arm as part of a bigger deal.




At some point in the past the original arm wand had become corroded, it was probably made in the early 80's, if not before. The previous owner, took the arm apart, rebuilt it with new bearings and made a new wand using a blue arrow shaft. A very good idea, and a nice piece of lateral thinking, but unfortunately, the wand is way too light, not enough effective mass. He then put the arm on the shelf and forgot about it.

I got the arm as part of a deal, the previous owner didn't want to see it go to waste.

A little later I bought a 401 and thought the two would make a good match.

But the 401 looked scrappy in bit of IKEA worktop. So I saved up and bought a Slatedeck that could hold up to a 12" arm.

To this I added a Slatedeck long-universal arm mount, this is an improvement over the existing arm base allowing better VTA setting and support for longer arms etc.

But the arm is still not right, I have tried adding mass, but the tube is so very lightweight to start with.

Next, I took the arm to j7 of AudioOrigami to see what could be done about the wand. He agreed to make a new wand, for a reasonable price, but being an awkward sole I thought, if I'm going/gone to all this expensive to resurrect this little beauty then why not do a 12" arm.

j7 confirmed my thoughts on the low effective mass and suggested I do some maths to work out what I want in terms of mass and geometry. He also asked for a scaled drawing to work from - I know I would.

So its time to get my maths dusted down, first up, the geometry.

Attached is a gif of a sketch of the geometry I would like j7 to make for me. I hope to eventually turn this into an scaled drawing for the wand, the Slatedeck base and the pillar from the arm will be reused.

Can anyone cast a critical eye on this to see if I have the correct starting point. I would like a 300m effective length and used the Loefgren "B" calculations from the BaerwaldLofgren spreadsheet to get the drawing attached.



thanks,

-- Andrew

[AndrewL]

Here's another good shot of the arm as it currently stands..



-- Andrew

[audioorigami]

and to make the maths harder the arm tube is offset from the main bearings...

[AndrewL]

Yeah, I was working up to that bit...

[JaS]

I would like a 300m effective length

Wouldn't we all

I haven't seen that spreadsheet, but the I calculated the figures for an effective length of 300mm with Loefgren null points as:

effective length: 300 mm
mounting distance: 286.015 mm
overhang 13.985 mm
offset angle: 18.149 degrees
linear offset: 93.445

Regards,
JaS

[AndrewL]

Thanks,

The spreadsheet I used is here...

http://www.ispexperts.com/BaerwaldLofgren.xls

The numbers are very close.

Can you tell from the image if I translated/rendered the numbers correctly into the geometry?

much appreciated,

-- Andrew

[AndrewL]

You can see the offset from the pivot j7 mentioned from this image.

http://www.vinylengine.com/phpBB2/album ... ic_id=1107

-- Andrew

[JaS]

Hi,
It's a bit difficult to read the figures (it looks like you have two offset angles?) but the basic drawing seems OK. This is the relationship of the dimensions:
http://www.feickert.com/dtsch/bedienung ... _klein.gif

Most straight tonearms have the tube offset from the bearings to reduce changes in cartridge azimuth as the tonearm rises and falls (this isn't shown in the drawings and isn't part of the lateral geometry).

While this doesn't affect the triangle formed by the effective length, angular offset and linear offset from the chosen formula, you may want to alter the headshell design to take this into account?

Regards,
JaS

[odaey]

Hi AndrewL, Audioorigami, Jas

I've been waiting for this to happen for quite some time and really hope I can hitch a ride on this journey.

I too am using an odyssey arm on a garrard GB301 and a woodbodied(uwe) denon 103. I'm pretty much useless and can't offer any help with regards to the math involved but i would love to have a copy of that 12 inch wand you guys are working on.

I've tried to give it some thought before but since the odyssey armwand is offset from the main pillar, getting the correct zenith and maintaining pivot to spindle distance by adjusting the eccentric ring on the base still puzzles me.

Always wondered if slots on the cartridge mounting area would have made things easier.

By the way, will this armwand be straight or curved?

regards,
odaey

[AndrewL]

Hi,
It's a bit difficult to read the figures (it looks like you have two offset angles?) but the basic drawing seems OK.

Sorry, yes, I was trying to choose a good size for the forum, it needs to be bigger.

This is the relationship of the dimensions:
http://www.feickert.com/dtsch/bedienung ... _klein.gif

Thanks, that is a nice clear concise visual description..

Most straight tonearms have the tube offset from the bearings to reduce changes in cartridge azimuth as the tonearm rises and falls (this isn't shown in the drawings and isn't part of the lateral geometry).

I wonder, I think you are refereng to vertical offset where the arm is above, or even below, the vertical pivot, do I understand correctly?

I this case its, the problem, as alluded to by j7, is looking from above, so in the horizontal/lateral plane. If you look from above, the root of the wand is to the left hand side of the arm's pivot point.

I hoped the image in the gallery would show where the 'root' of the arm wand is offset to one side of the pivot when looking from above. Perhaps I need to draw it?

While this doesn't affect the triangle formed by the effective length, angular offset and linear offset from the chosen formula, you may want to alter the headshell design to take this into account?

Regards,
JaS

Now that's a very valid point and one that j7 suggested could be a good way forward.

-- Andrew

[AndrewL]

Hi AndrewL, Audioorigami, Jas

I've been waiting for this to happen for quite some time and really hope I can hitch a ride on this journey.

You are more than welcome...providing j7 can oblige and my maths holds up.

I too am using an odyssey arm on a garrard GB301 and a woodbodied(uwe) denon 103. I'm pretty much useless and can't offer any help with regards to the math involved but i would love to have a copy of that 12 inch wand you guys are working on.

Nice set up...

I've tried to give it some thought before but since the odyssey armwand is offset from the main pillar, getting the correct zenith and maintaining pivot to spindle distance by adjusting the eccentric ring on the base still puzzles me.

I believe, this be viewed as the same as a more normal slider type arrangement. Happy to be corrected however that's how I've been using it.

By the way, will this armwand be straight or curved?

regards,
odaey

Straight is the plan, basically a copy of the existing wand for 300mm effective length. 16-18 gms effective mass, ideal for a 103 et al. Wire out the middle of the tube, and I hope j7 will be able foam fill.

You could even get j7 to give it some John Gordon style bling...

-- Andrew

[JaS]

I wonder, I think you are refereng to vertical offset where the arm is above, or even below, the vertical pivot, do I understand correctly?

That's another issue altogether Checkout the owners manual for the Audio Technica AT1010 for some theory behind the vertical offset.

I'm actually referring to the angle of the tube as it leaves the bearing block - by optimising this you ensure that as the cartridge rises and falls the stylus remains vertical.

You should be able to see the angle from these images, and it has no effect on the lateral alignment, it's purely to correct the vertical azimuth and line up the movement of the vertical bearing with that of the cantilever. My only concern with lengthening the arm tube is that you are loosing this relationship.









Regards,
JaS

[audioorigami]

this arms weird offset is like nothing any other arm is made like

the bearing and arm wand are not on the same plane ...and the headshell seems to be a weird angle and not inline with the arm tube

i have asked Seb to take a look at the maths
he wishes that you take pictures of the arm from above andrew and maybe close ups and the bearing and the headshell as well


bw
j7

[Seb]

andrew,

your problem is interesting

best regards

Seb